Ultrasonic Substrate Vibration-Assisted Drop Casting ... · A simple, low-cost, versatile, and potentially scalable casting method is proposed for the fabrication of micro- and nano-thin

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Ultrasonic Substrate Vibration-AssistedDrop Casting (SVADC) for the Fabrication ofPhotovoltaic Solar Cell Arrays and Thin-FilmDevicesMorteza Eslamian* and Fatemeh Zabihi

Abstract

A simple, low-cost, versatile, and potentially scalable casting method is proposed for the fabrication of micro- andnano-thin films, herein termed as ultrasonic “substrate vibration-assisted drop casting” (SVADC). The impingementof a solution drop onto a substrate in a simple process called drop casting, usually results in spreading of the liquidsolution and the formation of a non-uniform thin solid film after solvent evaporation. Our previous and currentsupporting results, as well as few similar reports by others, confirm that imposing ultrasonic vibration on the substratecan simply convert the uncontrollable drop casting method into a controllable coating technique. Therefore, theSVADC may be used to fabricate an array of emerging thin-film solar cells, such as polymer, perovskite, andquantum-dot solar cells, as well as other small thin-film devices, in a roll-to-roll and automated fabrication process. Thepreliminary results demonstrate a ten-fold increase in electrical conductivity of PEDOT: PSS made by SVADC comparedwith the film made by conventional drop casting. Also, simple planar perovskite solar cells made here using SVADCshow promising performance with an efficiency of over 3 % for a simple structure without performing processoptimization or using expensive materials and treatments.

Keywords: Solution-processed solar cells, Perovskite solar cells, Organic solar cells, Drop casting, Ultrasonic substratevibration, Process scale-up

BackgroundThe motivation behind the development of emergingphotovoltaic thin-film solar cells, such as polymer, per-ovskite, and quantum-dot solar cells, is their potential tobe fabricated using solution-processed molecular semi-conductor materials and vacuum-free scalable castingmethods. Among casting methods, spin coating is widelyused, but its application is limited to lab-scale and batchprocesses. Dip coating and doctor blading are other lab-scale casting methods, which are less controllable. Or-ganic solar modules have been fabricated in pilot scaleusing a combination of some of the scalable methods [1,2]. A scalable casting technique should be ideally, fast,low-cost in terms of solar cell materials and energy con-sumption, roll-to-roll fabrication compatible, touch-free,

automated, controllable, and suitable to coat small areasin an array of cells. The major potentially scalable cast-ing techniques currently under investigation for the fab-rication of emerging solar cells include the inkjetprinting [3], slot-die casting [4], screen printing [5],gravure printing [6], spray coating [7, 8], etc. Each of theabove-mentioned scalable techniques is usually suitablefor the fabrication of films with particular characteristics.For instance, inkjet printing can direct small droplets tothe target to make dots, lines and thin films. It is, how-ever, a rather low-throughput process, given that the fea-tures are fabricated based on impingement andcoalescence of many individual small droplets. Slot-diecasting method is not suitable for the fabrication ofnano-thin films, given that a rather large amount of so-lution is delivered during the deposition process, makingthe films thick. And while spray coating is a fast andscalable technique, it generally suffers from unsteadiness,

* Correspondence: [email protected] of Michigan-Shanghai Jiao Tong University Joint Institute,Shanghai 200240, China

© 2015 Eslamian and Zabihi. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made.

Eslamian and Zabihi Nanoscale Research Letters (2015) 10:462 DOI 10.1186/s11671-015-1168-9

inadequate film uniformity and intactness [8, 9]. Photo-voltaic thin-film solar cells or modules are usually madeof an array of small cells which are interconnected inseries and parallel to achieve the required voltage andcurrent [1, 2]. The size of each cell should be kept smallto achieve a higher device performance and efficiency,given that large-area thin films are more susceptible tomanufacturing defects and imperfections compared tosmall-area films. Therefore, masks may be required tobe used during the spray deposition and other castingprocesses to ensure that only specific areas are cov-ered, resulting in wastage of solar cell materials. Toimprove the uniformity of spray-on films, we have re-cently developed, implemented, and tested a modifiedcoating technique, termed as substrate vibration-assistedspray-coating (SVASC), in which during the sprayingprocess, controlled ultrasonic vibration is imposed on thesubstrate [10–12]. The imparted mechanical energy to thewet film leads to improvement in its uniformity as well itsnano-structure and functionality. The imposed substratevibration is also applicable to other casting methods, andhas been used by others as well, such as for the fabricationof transistors [13] and nanowires [14]. It has been showntheoretically [15, 16] and experimentally [11, 12] that theimposed substrate vibration can improve or deterioratesurface wetting, depending on the amplitude, power andfrequency of the imposed vibration. In the following sec-tions, a manufacturing technique is proposed for the large

scale fabrication of solar cell arrays that demonstrates con-tinuous fabrication of solar cell layers on a solar cell mod-ule using drop casting combined with imposed substratevibration, in a method herein termed as substratevibration-assisted drop casting (SVADC).

Presentation of the HypothesisConventional drop casting is not considered as a filmformation method, due to the lack of adequate controlover the film characteristics. However, drop casting whencombined with imposed ultrasonic substrate vibration canbecome controllable. In the proposed process, a small vol-ume of a precursor solution with low surface tension is re-leased from a capillary tube by dripping with negligibleinitial velocity or it is injected with an initial velocity and

Fig. 1 Drop impingement and spreading on a stationary substrate (left) and a vibrating substrate (right)

Table 1 Thickness and roughness of PEDOT: PSS films made byregular drop casting and substrate vibration-assisted drop casting;vibration time = 60 s

Case#

Vibrationpower (W)

Free fall height(cm)

Film thickness(μm)

Film roughness(μm)

1 0 2 18.2 8.98

2 0 10 10.3 5.35

3 5 2 2.93 2.09

4 5 10 2.41 1.86

5 20 2 1.80 0.90

6 20 10 2.00 0.86

Eslamian and Zabihi Nanoscale Research Letters (2015) 10:462 Page 2 of 5

is directed toward the substratet. In either case, the dropimpinges on the substrate with a velocity and momentum.The drop momentum results in substantial liquid spread-ing on the substrate. A wetting substrate with low inter-facial tension and a high drop impinging velocity facilitatedrop spreading. This is usually analyzed in terms of theWeber number (We), which is the ratio of the drop inertiaforce to the surface tension force. The drop Reynoldsnumber (Re) also represents the ratio of the drop inertiato the viscous force. Therefore, the solution viscosity,density, surface tension, drop size, and impinging velocityvia Re and We numbers, as well as surface wetting proper-ties control drop spreading. If the surface is not suffi-ciently wetting and Re and We are not large enough, thewet film may retract and recede resulting in the formationof a non-uniform thin solid film after drying. An exces-sively large Re and We may cause drop splashing which isan unwanted phenomenon [8, 17]. To improve dropspreading and uniformity of the deposited film, the pro-posed SVADC method takes advantage of imposedultrasonic substrate vibration with controlled vibrationpower (amplitude) and frequency. Another part of thenovelty of this work is the way that the SVADC can beemployed in an automated fabrication process, which isexplained later.

Testing the HypothesisPart of the hypothesis is backed by our previous studieson the effect of the imposed substrate vibration on top-ography and nano-structure of spray-on films. In onestudy, it has been observed that the uniformity, roughness,and electrical conductivity of spray-on PEDOT: PSS filmsignificantly improves, if the substrate is subjected to alow-power ultrasonic vibration [11, 12]. Also, according toour unpublished data, the post treatment of wet spun-onPEDOT: PSS films by imposed substrate vibration resultsin up to twelve-fold increase in electrical conductivity,caused by an enhancement in the film internal and surface

uniformity. The positive effect of the imposed substrate vi-bration on the film topography and the principle of oper-ation of SVADC are schematically shown in Fig. 1. Todemonstrate the effectiveness of the SVADC method, pre-liminary experiments were performed on 4-mm-sized freefalling dripping drops of 1.3 wt.% PEDOT: PSS aqueoussolution diluted with IPA at a volume ratio of 4:1, re-spectively, impinged on a smooth glass substrate fromthe above. In some cases, the glass substrate was placedatop a metal box attached to an ultrasonic transducer(Cheersonic Ultrasonic Co., China). The transducer wasconnected to a signal generator operating at a fre-quency of 40 kHz with variable power or vibrationamplitude. More details may be found in Ref. [12]. Theas-casted wet films were then dried and annealed for25 min at 120 °C in an oven. Samples were analyzedusing a confocal laser scanning microscope (CLSM;Zeiss, model LMS700, Germany). The CLSM-measuredthickness and the area-averaged RMS roughness ofsome samples on an area of 600 × 700 μm are shown inTable 1. The results manifest that the application of im-posed vibration results in significant reduction in filmroughness and film thickness due to better drop spread-ing. Also, a larger free fall height (distance between thesubstrate and the initial position of the drop) results inlower roughness and thickness due to increased dropvelocity and momentum at the time of impact. How-ever, the free-fall height has nearly no effect if thepower of the imposed ultrasonic vibration on the sub-strate is as high as 20 W, indicating that the substratevibration is a strong factor controlling the film character-istics. The laser images of sample 2 (casted on stationarysubstrate with no vibration) and sample 6 (casted on vi-brating substrate at 20 W) are shown in Fig. 2. It is ob-served that the imposed substrate vibration (Fig. 2b)results in the uniform distribution of black PEDOT: PSSgrains within the film and reduced number of pinholesand imperfections. The rupture is clearly seen in the film

Fig. 2 Laser images of the PEDOT: PSS films made by drop casting. a Case 2 of Table 1 (stationary substrate). b Case 6 of Table 1 (vibrating substrate).Electrical conductivities are shown on the images. The image area is 600 × 700 μm

Eslamian and Zabihi Nanoscale Research Letters (2015) 10:462 Page 3 of 5

made using conventional drop casting as shown in Fig. 2a.The electrical conductivity of the films, measured usingthe four-point probe method along a 6 mm straight lineon the films [11] provided on the images of Fig. 2, showsan impressive ten-fold increase when ultrasonic vibrationis imposed on the substrate (3.35 S cm−1 for the non-vibrating substrate versus 34.3 S cm−1 for the vibratingsubstrate). The slope of the I–V curves of the films, whichare linear (not shown), and the film thickness were usedto obtain the film electrical conductivity [11].Careful evaluation of the performance of the SVADC

method, however, entails design of well-thought experi-ments, fundamental studies, and device fabrication andtesting. Here just to demonstrate the merit of the SVADCmethod, a single planar mixed-halide perovskite solar cellwas fabricated based on the simple planar FTO-coatedglass/PEDOT: PSS/CH3NH3PbI3 − xClx/PCBM/Al archi-tecture. The IPA solvent was added to PEDOT: PSS aque-ous solution (1.3 wt.% PEDOT: PSS dispersed in water)with the volume ratio of 3:1, respectively. The PEDOT:PSS solution was dropped onto FTO-coated glass, form-ing a film which was annealed on a hot plate at 120 °C for

20 min. This is the hole-transporting layer of the cell. Toprepare perovskite (active or light harvesting layer) re-agents, MAI was dissolved in 2-propanol to obtain a solu-tion with a concentration of 0.1 mg mL−1, and PbCl2 wasdissolved in a 3:1 volume ratio of the mixture of DMSOand DMF solvents to obtain a solution with a concentra-tion of 0.26 mg mL−1. PbCl2 solution was drop-castedatop PEDOT: PSS film, and the formed crystallinePbCl2 film was annealed at 90 °C for 30 min; then MAIsolution was dropped atop the PbCl2 film. MAI solu-tion reacts with PbCl2 film, making the mixed-halideperovskite CH3NH3PbI3 − xClx film. The perovskite layerwas annealed at 90 °C for 100 min. PCBM powder wasdissolved in chlorobenzene forming a 50 mg mL−1 solu-tion, and was dropped on the CH3NH3PbI3 − xClx layerand annealed at 80 °C for 15 min. PCBM layer helps ex-traction and transport of electrons to the back contact.All layers were made by conventional drop casting, aswell as SVADC in a glovebox at a vibration power of5 W at 90 s, except the Al back contact, which was de-posited by thermal evaporation. All chemicals werepurchased from Sigma-Aldrich, USA.The current density-voltage curves and PCE of solar cell

devices were obtained by a solar simulator and a sourcemeter (National Instruments, model NI PXI-1033, TX,USA) under AM1.5G solar irradiation with intensity of1000 W/m2. The cells made using regular drop castingshowed no or only a weak output. Figure 3 shows thecurrent density-voltage of a cell (2 × 2 mm) made bySVADC with a PCE of 3.02 % in a cell made with minimumprocess optimization and using a very simple but scalableprocess. This efficiency is lower than the efficiency of asomewhat similar device made by spin coating andspray coating (active layer only) [18]. In Ref. [18], thethickness of each layer was optimized and Ca/Al wasused as the back contact instead of Al, to improve theenergy band diagram and to facilitate charge collection.Therefore, performing process optimization on eachlayer and using Ca/Al as the back contact can further

Fig. 4 Schematic of proposed automated manufacturing apparatus incorporating SVADC for the fabrication and heat treatment of solar cell arrays

Fig. 3 Current density-voltage curve of a planar FTO-coated glass/PEDOT: PSS/CH3NH3PbI3 − xClx/PCBM/Al perovskite solar cell made bysubstrate vibration-assisted drop casting (SVADC)

Eslamian and Zabihi Nanoscale Research Letters (2015) 10:462 Page 4 of 5

improve the short-circuit current density, open-circuitvoltage, fill factor, and therefore the efficiency of the cellmade by SVADC. Device optimization process was be-yond the scope of this work.

Implication of the HypothesisThe proposed coating technique, SVADC, may be usedfor the fabrication of small-area coatings and thin-filmdevices in an automated process as well. One applicationis the photovoltaic solar cell arrays, made of severalsmall cells fabricated on a panel moving by a conveyoras shown in Fig. 4. One or more nozzles or capillarytubes release one or few drops of a solar cell precursorsolution onto small-area vibrating substrates to formvarious layers of thin-film solar cells, successively. Theas-casted wet film may be exposed to an irradiativeheater for film drying and post annealing, a process toenhance the film nano-structure. The process is roll-to-roll compatible if flexible substrates are used. Also, severallayers of a thin-film solar cell may be deposited sequen-tially using the same manufacturing process. The max-imum effective and uniform surface area that can beobtained for application in a device depends on the sur-face wettability, solution properties, impingement con-ditions, and substrate vibration.

AbbreviationsCLSM: confocal laser scanning microscope; DMF: N,N-dimethylformamide;DMSO: dimethyl sulfoxide; FF: field factor; FTO: fluorine-doped tin oxide;IPA: isopropyl alcohol; Jsc: short-circuit current density; MAI: CH3NH3I;PCE: power-conversion efficiency; PCBM: phenyl-C61-butyric acid methylester; PEDOT: PSS: poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate);RMS: root-mean-square; SVADC: substrate vibration-assisted drop casting;Voc: open-circuit voltage.

Competing InterestsThe authors declare that they have no competing interests.

Authors’ ContributionsM.E. conceived the idea of SVADC for large-scale fabrication of solar cellarrays and wrote the paper. F.Z. performed the current experiments, madeand tested the device, and characterized the films. F.Z. also significantlycontributed to the development of the substrate vibration-assisted spraycoating technique introduced in previous works. Both authors reviewedand confirmed the paper.

Authors’ InformationM.E. is an Associate Professor of Mechanical Engineering at the University ofMichigan-Shanghai Jiao Tong University Joint Institute. His research focuseson thermal-fluid sciences, energy conversion, and nanotechnology and inparticular on thin-film solar cells. F.Z. is a senior Postdoctoral Research Associateat the University of Michigan-Shanghai Jiao Tong University Joint Institute. Hercurrent research focuses on the fabrication of perovskite solar cells by substratevibration-assisted coating techniques.

AcknowledgementsThe research fund by the Shanghai Municipal Educational Commission grantedto M.E. under the 2014 Oriental Scholar program is acknowledged.

Received: 14 October 2015 Accepted: 23 November 2015

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